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Implementing Finite State Automata with Digital Circuits

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• A Finite State Automata (FSA) is a device that can be in a finite set of states and can move from one state to another depending on a set of (external) input signals.

• I will present the design of a very common finite state automatum: a traffic stop light.

  A (live) traffic stop light can be in one of 4 states:

  • Red
  • Green
  • Yellow
  • Error (blinking)

  A traffic stop light has internal circuitry that do "self-test" and this circuitry would output either Error = 0 (no error detected) or Error = 1 (error detected). When the self-test detects an error, the traffic light would enter the "Error" (blinking) state. Otherwise (no error or a normally functioning traffic light), the traffic light would continuously go through the states Green, Yellow, Red, Green, Yellow, Red, and so on.

  The following diagram is a state transition diagram for the traffic stop light:

  

• The state transition diagram fully specify the behaviour of a FSA.

  So the first step in designing a digital circuit that implements a FSA is to represent the states in the FSA's state transition diagram by numbers.

  We simply number each state of the diagram. There are 4 states in the traffic stop light FSA and I have assigned the states as follows:

  • Red = state 0 (00 binary)
  • Green = state 1 (01 binary)
  • Yellow = state 2 (10 binary)
  • Blinking = state 3 (11 binary)

  The state diagram becomes:

  

• Let me list a number of facts that will help you understand how to construct a digital circuit that implements the above FSA.

• Fact 1:
  • The FSA need to remember its current state
  • We have learn about 2 digital circuit that can "remember":
    • D-latch
    • D-flipflop
  • You will see below that the D-latch cannot be used
  • Since we have 4 states, we will need two bits to represent the 4 different states.
  • Very important: The outputs of the memory devices will represent the state of the FSA

• Fact 2:
  • The way you write/update a D-flipflop (and D-latch) is as follows:
    • You first present the value that you want to store in the D-flipflop to the input of the D-flipflop
    • Generate a clock/write signal to the D-flipflop's clock input

• Fact 3:
  • The FSA moves from one state to another state
  • The state to which the FSA moves depends on
    • the error indication signal E
    • the current state
  (The above can be concluded by looking at the state transition diagram)

• Schematically, the FSA will look this:

  

  • It will have 2 memory elements to remember 4 possible states
  • In the figure, the outputs of the 2 memory elements are Q₁ and Q₀
  • We will need 2 circuits (C₁ and C₀) to form the value D₁ and D₀ for the memory elements
  • The value D₁ and D₀ will be written into the memory elements at the next clock signal

• Now I can explain why we cannot use D-latches for memory elements to construct the FSA:
  • Due to fact that the output of the memory element is used as input to the memory element, the write period must be extremely short, or else the new output value will travel back to the input and form another result which will travel to the output again and form yet another result, and so on and so on.
  • The resulting circuit is not stable.

• The circuits C₁ and C₀ has 3 inputs and are designed using the state transition diagram as follows:

   

  You can construct a (combinatorial) circuit from this table using the circuit design technique described in this webpage and re-enforced in Project 1...

  Or, even easier, you may observe that:

  • D1 = E or (not-Q₁ and Q₀)
  • D0 = E or (not-Q₁ and not-Q₀)

  And use these two relations to construct circuits C₁ and C₀ in the above figure.

  Here is a partial ciruit diagram (showing only the circuit for D1):

          

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• I have built the circuit in the following logic-sim circuit file for you to experiment:

  /home/cs355001/bin/cs355sim /home/cs355001/demo/circuits/seq-circuit1-demo

  The following circuit is the same as the previous one, except I have added probes that show the input values to the D-flipflops. This circuit will clearly show how the FSA moves from one state to another:

  /home/cs355001/bin/cs355sim /home/cs355001/demo/circuits/seq-circuit1

  Finally, in the following circuit, I have added a "display" circuit that translates the state into lighting patterns of a traffic stop light:

  /home/cs355001/bin/cs355sim /home/cs355001/demo/circuits/seq-circuit1-demo-plus-light